1. Field of the Invention
This invention relates to a method and apparatus for color area compression of color video data which may be applied with advantage to, for example, a desk top publishing (DTP) system centered about a work station.
2. Description of the Related Art
In general, the color range, that is a color reproducing range or gamut, of a device handling a color image, differs from device to device. Since a monitor achieves color reproduction by additive mixing by coloration of three prime colors of red (R), green (G) and blue (B), the color gamut of the monitor is determined by the types of the fluorescent material employed. On the other hand, a printer reproduces the color by the cyan (C), magenta (M), yellow (Y) and black (B) of the ink. The color gamut of the printer differs not only with the ink employed but also with the sort of the paper sheet as the image reproducing medium and with the form of gradation representation.
Referring to FIG. 1 showing the result of integration in the direction L* in an a*-b* plane of the color gamut of a monitor of computer graphics (CG) and the color gamut of an ink jet printer, the color gamut CM.sub.ijp 3 of a printer is lower than the color gamut CM.sub.mon of the monitor. Above all, color reproducibility of the high chroma area of green (G) and blue (B) is extremely low. As for other color hues, for which no clear distinction can be made in FIG. 1, the peak of chroma is deviated in the direction of lightness. Consequently, color reproducibility from the monitor to the printer is not good especially in high lightness and in high chroma areas.
In a natural picture, high saturation (chroma) color occurs less frequently, while a color closer to unsaturated color is predominant. Since a CG picture is drawn based on a monitor output, colors of high saturation that cannot be regenerated by a printer are frequently used. The result is that the proportion of the colors outside the color gamut is higher in a CG image so that the color reproducibility in the CG image is low as compared to the case of printing a natural picture. Heretofore, a device handling a color picture is used under such an environment that it is connected to a specified input/output device, such as a camera or a monitor handling red (R), green (G) and blue (B) or luminance (Y) and color differences (U), (V) or (I), (Q), or a scanner or a printer handling cyan (C), magenta (M), yellow (Y), black (K) or red (Dr), green (Dg) and blue (Db). Under such environments, closed color correction between the input/output devices is performed, while the technique of color gamut compression can be determined solely by the color range or color gamut of the input/output device.
However, in an environment to which many users connect their various devices, close color correction between the input/output devices cannot be employed.
Thus a demand has been raised for the concept of a device independent color regeneration technique of representing a picture with the similar color regardless of input/output devices types. The system for realization of a device independent color regeneration is generally termed a color management system (CMS).
In CMS, when connecting various input/output devices, such as a camera 61, a scanner 62, a monitor 63 or a printer 64, as shown in FIG. 2, color signals of an input system are converted into color signals of an output system at a time in a device independent common color space, such as CIE/XYZ or CIE/L* a* b* by a conversion formula or a conversion table known as profile.
If, in the CMS, the color gamut of an output system is larger than that of the input system, the picture can be directly outputted so that no problem is incurred. Conversely, if the color gamut of an input system is larger than that of the output system, the color information cannot be directly regenerated correctly, depending on the image type. For example, if an image on a monitor is to be outputted to a printer, the color outside the gamut of the printer cannot be directly regenerated. In such case, such color correction is required in which the original picture information, such as gradation or color hue, is maintained as far as possible and the color outside the gamut is brought into the gamut. The technique of forcing the physically non-reproducible colors into the gamut by some means or other is generally termed gamut compression.
In connection with the color perception by the eyes, the color has three attributes, namely the lightness representing the color lightness, chroma representing the color brightness and color hue representing the color type. The color space which is based upon the three attributes of the color perception of the human being may be exemplified by the CIE/L* C* h color space, where L*, C* and h represent lightness, chroma and color hue, respectively. These three attributes can be handled as independent parameters.
The gamut compression may be performed in the CIE/L* C* h color space for utmost perceptual comprehensibility. The generally accepted practice is to perform color compression in a two-dimensional plane of lightness L and chroma C, with the color hue h being kept constant.
Among conventional techniques for gamut compression, there are a chroma compression method in which lightness L* is kept constant and only chroma C* is compressed, as shown in FIG. 3, a lightness compression method in which chroma C* is kept constant and lightness L* is compressed, and a minimum color difference method of minimizing the color difference in the L*-C* plane, as shown in FIG. 5.
In the conventional chroma compression method, the resulting picture generally lacks vividness, although gradation of the high chroma area is maintained to a limited extent.
In the lightness compression method, while chroma is scarcely lowered, difference in lightness becomes more pronounced the higher becomes the saturation. Since the high lightness area is compressed to a low lightness area and the low lightness area is compressed to a high lightness area, an area shown shaded in FIG. 4 becomes of the same color.
In the minimum color difference method, the color difference becomes minimum, so that a picture mathematically closest to the original color is obtained. However, an area shown shaded in FIG. 5 becomes of the same color.